Electrolysis of Water Experiment

Electrolysis of water experiment is a classic chemical experiment, which is of great significance for understanding the composition of water and the principle of electrolysis. The following is a detailed introduction:

Purpose of the experiment

Through the method of electrolysis of water, prove that water is composed of hydrogen and oxygen elements; understand the basic principles and processes of electrolysis reaction; observe the electrode reaction phenomenon and the properties of the generated gas.

Principle of the experiment

Water decomposes under the action of direct current, and its chemical equation is: 2H₂O = power on = 2H₂↑ + O₂↑. In the electrolytic cell, the cathode connected to the negative pole of the power supply undergoes a reduction reaction, and the hydrogen ions (H⁺) in the water gain electrons to generate hydrogen; the anode connected to the positive pole of the power supply undergoes an oxidation reaction, and the hydroxide ions (OH⁻) in the water lose electrons to generate oxygen and water.

Experimental device

Power supply: Generally, a DC power supply is used, such as a battery or a DC regulated power supply, and the voltage is usually 6-12V to provide stable DC power.
Electrolyzer: You can use a special electrolytic device such as a Hofmann electrolyzer, or you can make a simple electrolytic cell yourself. A simple electrolytic cell can be made using a U-tube or a larger beaker or other container.
Electrodes: Graphite rods or platinum sheets are commonly used as electrodes. Graphite electrodes have low cost, good conductivity, and relatively stable chemical properties; platinum electrodes have high catalytic activity and are not easily oxidized, but they are more expensive. If metal electrodes (such as iron, copper, etc.) are used, the electrode itself may undergo oxidation reactions at the anode, interfering with the experimental results.
Electrolyte: Generally, dilute sulfuric acid (H₂SO₄) or sodium hydroxide (NaOH) solution is used. Taking dilute sulfuric acid as an example, sulfuric acid is completely ionized in water to produce hydrogen ions (H⁺) and sulfate ions (SO₄²⁻), which increases the conductivity of the solution and is conducive to the electrolytic reaction; when sodium hydroxide solution is used, its ionization of sodium ions (Na⁺) and hydroxide ions (OH⁻) can also play a similar role. However, it should be noted that the concentration of the electrolyte should not be too high to avoid danger.

Experimental steps

Preparation: Check the airtightness of the device and whether the electrode connection is correct. Inject an appropriate amount of electrolyte (such as dilute sulfuric acid or sodium hydroxide solution) into the electrolytic cell so that the electrode is completely immersed in the electrolyte.
Connect the power supply: Connect the DC power supply to the electrode of the electrolytic cell correctly, turn on the power supply, and start electrolysis. It can be observed that bubbles are generated on the surface of the electrode. The bubble generation rate of the two electrodes is different. The cathode connected to the negative pole of the power supply generates bubbles at a fast rate, and the anode connected to the positive pole of the power supply generates bubbles at a slow rate.
Collecting gas: The generated gas can be collected using the drainage method. Invert a small test tube or gas collecting bottle filled with water above the two electrodes. As the gas is generated, the water in the small test tube or gas collecting bottle is discharged. When a certain amount of gas is collected, cover the mouth of the gas collecting bottle with a glass sheet under water and take it out of the electrolytic cell.

Experimental phenomenon and analysis

Cathode phenomenon: The volume of the collected gas is about twice the volume of the anode gas. The gas collected at the cathode is tested with a lit wooden stick. The gas can burn and produce a light blue flame, which proves that it is hydrogen. This is because at the cathode, hydrogen ions (H⁺) gain electrons to generate hydrogen. According to the chemical equation for electrolysis of water, the volume ratio of hydrogen to oxygen is 2:1.
Anode phenomenon: The gas collected at the anode can rekindle the wooden stick with sparks, proving that it is oxygen. This is because at the anode, hydroxide ions (OH⁻) lose electrons to generate oxygen and water.

Experimental precautions

Safety issues: Be careful to prevent electric shock when using a DC power supply; if dilute sulfuric acid is used as an electrolyte, avoid sulfuric acid splashing on the skin and clothing. If accidentally contacted, rinse immediately with plenty of clean water; if sodium hydroxide solution is used, also pay attention to its corrosiveness.
Experimental condition control: Keep the electrode clean to avoid the electrode surface being covered with oil or impurities, which will affect the electrolysis effect; the concentration of the electrolyte should be appropriate, too high or too low may affect the experimental results or cause the experiment to fail; during the experiment, a stable power supply voltage should be maintained to avoid voltage fluctuations from interfering with the experiment.

Through the water electrolysis experiment, we can intuitively understand the composition of water and the principle of electrolysis reaction, laying the foundation for further learning of chemical knowledge.

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